Campbell Scientific SDM-SIO1 Serial Input/Output Module - Manual (Page 14)

SDM-SIO1 Serial Input/Output Module
As shown above in TABLE 3-3 connections 1 to 5 need to be made to the
datalogger. SDM connections are made to appropriate logger control ports or to
the dedicated SDM port when fitted, e.g. the CR3000. The connection of the
wires to the remote serial device will vary with type of device and method of
communication. It is necessary to work out the best mode of operation of the
serial device, taking into consideration issues such as power consumption,
cable lengths (RS-485 being better than RS-232), synchronization of data
collection etc.
In Section 3.3, Examples for Connecting the SDM-SIO1 to Other Equipment,
there are some examples of different connection schemes for the serial devices.
Further discussion of different modes of operation is given in Section 4,
Programming the Datalogger.
Do not connect both RS-232 and RS-485 interfaces to the SDM-
SIO1 at the same time as this may cause a bus contention and
even possibly cause damage. NOTE
3.2 Safety Considerations
The SDM-SIO1 is considered to be a component of a measurement system that
is installed in an enclosure and wired up in accordance with this manual. Due
to space considerations full details of the maximum ratings of the connections
are not given on the device. Instead the user should study this manual and in
particular Section 2.2.1, SDM-SIO1 Current Consumption, to determine the
maximum voltages that are applicable to any terminal before starting an
installation.
The RS-485 0V ground reference connection may be needed to insure all units
are referenced to a common ground voltage. This is more often needed with
long cable runs. It is advisable to check the difference in ground potential with
some caution before connecting any wires, to insure the potential differences
are reasonably close and excessive current will not flow between the two
ground wires. There is a current limiting resistor fitted in the 0V line in the
SDM-SIO1, but this will not be adequate in the event of a serious ground fault,
e.g. the ground references being 120 V apart, due to faulty AC wiring. If a
large potential difference is found please seek the advice of a qualified
electrician before continuing with the installation.
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